Russian prepositions extend and specify the meanings of the case system. For that reason, while cases may appear without prepositions, prepositions must be accompanied by a case. This means that prepositions in Russian may not appear alone, as adverbs or conjunctions, as they may in English. In English, all the following are grammatically acceptable:

English Preposition-Adverb-Conjunction

John saw me before the ball.

Preposition

John had seen me before.

Adverb

John had seen me before I met my wife.

Conjunction

In Russian only the first two constructions are possible, Preposition + Noun or Pronoun, either of which must bear the appropriate case ending.

Russian Prepositions, Etc.

Ваня видел меня до бала.

Preposition before

Ваня раньше меня видел.

Adverb before

Ваня видел меня до того, как я встретился с женой.

Conjunction before

Notice in the Russian table that the preposition до+Gen cannot serve as an adverb at all and when it serves as a conjunction, it must have a dummy pronoun, то bearing the genitive case marker, -ого. This is because conjunctions introduce entire sentences and sentences cannot bear case. Russian is forced to use dummy pronouns to reflect the case required by any preposition which is used as a conjunction.

The major linguistic principle to remember in connection with prepositions is this:

The Preposition Rule

Prepositions must always have an object noun or pronoun which bears the case governed by the preposition.

To help you remember this rule, and to learn the cases associated with the Russian prepositions, all prepositions will be presented with the case they govern in the discussion of them which follows. Moreover, they will be presented according to the case the govern, beginning with the first case, the nominative. (See Adverbs for more on the subject.)